Heavy brines are useful in a variety of applications, particularly as "clear brines" in oil wells drilling. These brines are used, e.g., instead of drilling mud and for well closure. Therefore, they must be both heavy and possess a high boiling point. Furthermore, they must be substantially free of precipitates, in order not to clog pores in the ground from which gas, oil and the like materials are liberated. Such brines normally have a density of the order of about 14-23 ppg (pounds per gallon). A density of 21 ppg represents, for instance, a Zn,Ca/Br.sub.2 brine with 75% ZnBr.sub.2. Such brines, which can be provided in different ZnX.sub.2 :CaX.sub.2 :NaX.sub.2 :KX.sub.2 ratios, (X representing Br or Cl) are known to be corrosive to carbon steel, its corrosive activity increasing with increasing Zn.sup.++ ion concentration. Throughout this specification the term "carbon steel" is intended to embrace also what is commonly known as "drilling steels".
High Zn.sup.++ brines are the most corrosive, because Zn.sup.++ requires low pHs and the corrosion mechanism of carbon steel is affected by the pH, the corrosion being enhanced by a reduction in pH. Such brines have a diluted pH of 6 or less. "Diluted pH" values are those values obtained after diluting a brine tenfold in water. This corrosive activity can be appreciated from Table 1 below, which exemplifies this effect. In order to study the corrosion effect of such brines, six different brines were prepared, using a commercial brine (ex Bromine Compounds Limited, Israel) with a density of 19.2 ppg, and mixing it in different proportions with a 52 wt % CaBr.sub.2 brine (density: 14.2 ppg). As will be apparent to a person skilled in the art, the density of the brine is a function of the ZnX.sub.2 :CaX.sub.2 :NaX.sub.2 :KX.sub.2 ratio, as well as of the different species (chlorides and/or bromides) present in the brine.